The objective of this study is to define the chemical nature of the specific interaction of collagen with the platelet membrane. This will involve the identification of the collagen "receptor(s)" in human platelet membrane and the characterization of the mechanism whereby multiple linked interactions of a collagen matrix with such receptors effect specific adhesion of platelets to collagen and induce secretion of the contents of platelet granules (ADP, serotonin, etc.) which in turn causes platelets to aggregate. Although it is well established that native fibrillar collagen is required for initiation of platelet aggregation, the role and nature of polyvalent interactions are incompletely defined. The specific aims of the proposal include: 1) The identification and characterization of the human platelet membrane component which interacts with collagen to initiate platelet aggregation. 2) The definition of the structural characteristics of the collagen matrix required for platelet adhesion and aggregation. 3) A study of possible relationships of platelet membrane collagen "receptor(s)" with thrombin sensitive platelet membrane proteins and other membrane glycoproteins. 4) Isolation of glycoprotein I/glycocalicin from platelets and exploratory studies of their structural inter-relationship, including their physical chemical behavior and their interaction with lipids and detergents.